It's very expensive. They Fed Ex it. We had a filet sent back home and it cost around $100 as I recall. That was a few years ago. See if your ship will cook it for you. We had one cruise where they did (Princess). Another one (RCCL) didn't.
Every fish processor I know of uses FedEx, and that's where the bulk of the expense comes from. Shipping overnight to Seattle, for example is far less than to NYC, so a lot of it depends on where you live.
Also, pay attention to what you have done with your fish. Fresh or flash frozen product MUST be shipped "next day" and incurs a higher cost to ship faster. Smoked or canned salmon is cooked and not as perishable. A reputable fish processor will ship the cooked products via 2nd day air, which shouldn't be nearly as costly as overnight.
A general rule of thumb I give my fishing clients is to plan on between $50 and $75 for about 10 pounds of product to arrive at your home. Obviously there are many variables here. A fishing charter when you're on a cruise is a great way to have fun and see the area, but it's not the most economical way to buy fish for your freezer at home. Non-cruise travelers have options other than FedEx for getting things home (carry a cooler, wax-box, etc.)
There are some new DEC/Health Department regulations having to do with what can be brought into a commercial kitchen (as on a cruise ship). Officially speaking, last year (2007) NO cruise ships allowed you to bring your fish back on board. If you're out with an experienced charter operator (one who takes good care of your catch) and if you make arrangements in advance (see the purser or ShoreEx desk) you CAN still get fish back aboard a few Princess ships, but it takes some special "connections."